Active research is being conducted for organic thin-film transistors (TFTs), as they have the following advantages.
1. High flexibility in terms of the variety in the materials, manufacturing method, and product format.
2. The area can be easily increased.
3. The layer structure can be simplified, and the manufacturing process can be simplified.
4. A low-cost manufacturing device can be used for the manufacturing process.
Examples of the film forming method of the organic semiconductor layer are a printing method, a spin-coating method, and a dipping method. An organic TFT can be manufactured at a considerably lower cost compared to a conventional TFT using a Si semiconductor material.
When integrating an organic TFT, it is necessary to form an electrode pattern. Patent document 1 discloses a method of manufacturing a laminated structure, including a step of forming a wettability varying layer including a material whose critical surface tension is changed by receiving energy; a step of forming a pattern with portions of different critical surface tensions including a low surface energy portion having a low critical surface tension that is formed by applying energy to a part of the wettability varying layer and a high surface energy portion having higher critical surface tension; a step of forming a conductive layer in the high surface energy portion by applying a liquid including a conductive material on the surface of the wettability varying layer on which the pattern is formed; and a step of forming a semiconductor layer on the wettability varying layer.
Furthermore, when fabricating an organic TFT, it is necessary to form a pattern of the organic semiconductor layer. If the organic TFT is integrated without forming a pattern of the organic semiconductor layer, the following problem may occur. That is, due to the impact of the organic semiconductor layer formed on portions other than the channel region, an off current may be generated while the transistor is operating, in which case power consumption will increase. Furthermore, this may cause crosstalk when displaying pixels. When fabricating a TFT using the Si semiconductor material, a pattern is formed with the Si semiconductor material by photolithography etching.
When consideration is made only for forming a pattern of the organic semiconductor layer, a photoresist is applied, a desired pattern is formed by a light exposure process and a developing process, thereby forming a resist pattern. This is used as an etching mask to perform etching. Then, the resist is peeled off, thereby forming the pattern. However, when a high polymer material is used as the organic semiconductor material, and a pattern is formed by applying a photoresist on the high polymer material, the transistor properties may be degraded. A photoresist is obtained by dissolving a novolac resin using naphthoquinone diazide as the photosensitive group in an organic solvent such as a xylene solvent and a cellosolve solvent. The high polymer material is often dissolved in an organic solvent included in the photoresist. When a crystalline molecule such as pentacene is used as the organic semiconductor material, the transistor properties may be similarly degraded to some degree or another. Furthermore, damage may be caused by using a peeling agent such as ethylene glycol monobutyl ether and monoethanolamine when peeling off the resist. Damage may be caused by rinsing the organic semiconductor layer with pure water after peeling off the resist. Due to the above reasons, it is difficult to form a pattern of the organic semiconductor layer by the conventional photolithography etching method.
Patent document 2 discloses a method of manufacturing a transistor, including a step of providing a conductive layer on a substrate; a step of providing a mask having at least one window on the conductive layer; a step of etching the conductive layer through the window to form an opening in the conductive layer, and setting part of the conductive layer to form the source and the drain of the transistor; a step of depositing the conductive material through the window and forming a metal transistor gate in the opening; a step of forming a dielectric layer made of a metallic oxide on the gate; and a step of putting a semiconductor material in between the source and the drain, on the gate, and in a space between the source or the drain and the gate, to thereby form the semiconductor body of the transistor. The etching is performed in such a manner as to cause undercutting at the periphery of the window, so that the opening is wider than the window in the direction parallel to the surface of the substrate. The conductive material is deposited by metal evaporation in such a manner that the periphery of the gate is spaced away from the source and the drain, and that the periphery of the gate of the transistor perfectly overlaps the periphery of the opening.
Patent document 3 discloses a method of fabricating an organic transistor by appropriately combining the following methods. One method is for applying charges to a predetermined position of surface to be coated as well as applying charges of a polarity opposite to that of the aforementioned charges to a coating material to attract the material applied with charges to a predetermined position with a Coulomb force. Another method is for forming a recessed part at the predetermined position of the surface to be coated to deposit the coating material on the recessed part. Yet another method is evaporating the solvent after applying the coating material to form the pattern, and then radiating a laser beam to the pattern.
However, the problem with these processes is that as the number of process steps increases, the throughput will decrease and the manufacturing cost will increase.
Patent document 4 discloses a thin film transistor including a gate electrode formed on a substrate; a gate insulating film formed on the gate electrode; a source electrode formed on the gate insulating film; a drain electrode formed on the gate insulating film; a semiconductor film made with an aggregate of organic semiconductor molecules, whereby the semiconductor film is formed on the gate insulating film, the source electrode, and the drain electrode; and a self-assembled monolayer formed inside the gate electrode projection region and at the interface between the gate insulating film and the semiconductor film. Furthermore, light is radiated onto the self-assembled monolayer formed on the surface of the insulating film, from the backside of the substrate with the gate electrode acting as a photomask, to remove the self-assembled monolayer from the region in which a semiconductor film is formed, other than the gate electrode projection region.
However, with this method, the organic semiconductor material is limited, and therefore the freedom in selecting the material is low.
Meanwhile, when a high polymer material that is soluble in an organic solvent is used as the organic semiconductor material, the pattern can be formed by the inkjet method. With the inkjet method, the pattern can be directly rendered, and therefore the material usage rate can be considerably enhanced. Furthermore, by performing the inkjet method to form a pattern, the manufacturing process may be simplified, the yield may be increased, and the cost may be decreased.
However, when forming a pattern on a large area, it is difficult to perfectly form a pattern for all transistors, due to factors such as the precision in points of impact. Particularly, physical properties of the organic semiconductor ink, such as viscosity, surface tension, and drying conditions change considerably due to the purity of the high polymer material, the amount of molecules, the molecule amount distribution, and the solvent. Thus, it is difficult to adjust the physical properties to appropriate levels. For this reason, ink cannot always be jetted properly from all of the nozzles. In some cases, the jetted ink from one of the nozzles may be deflected, or the amount of jetted ink may vary. The same applies to head properties, and the nozzles will not always have the same properties. When the jetted ink from one of the nozzles is even slightly deflected, a pattern may be formed properly at low resolution but not at high resolution. As a result, the organic semiconductor layer may be a partially incomplete pattern. Such a problem is particularly notable when forming patterns on a large area.    Patent Document 1: Japanese Laid-Open Patent Application No. 2005-310962    Patent Document 2: Japanese Patent Application Publication No. 2003-536260    Patent Document 3: Japanese Laid-Open Patent Application No. 2004-297011    Patent Document 4: Japanese Laid-Open Patent Application No. 2005-79560
Accordingly, there is a need for an organic transistor capable of decreasing the off current while the transistor is operating, an organic transistor array including a plurality of such organic transistors, and a display device including the organic transistor array.